As proof, Wikileaks has posted a 32-page classified document [PDF] from the Department of Defense Intelligence Analysis program, dated March 2008, which details "the counterintelligence threat posed to the U.S. Army by the Wikileaks.org Web site." It reads:

The possibility that a current employee or mole within DoD or elsewhere in the U.S. government is providing sensitive information or classified information to Wikileaks.org cannot be ruled out. Wikileaks.org claims that the "leakers" or "whistleblowers" of sensitive or classified DoD documents are former U.S. government employees. These claims are highly suspect, however, since Wikileaks.org states that the anonymity and protection of the leakers or whistleblowers is one of its primary goals.

If I'm parsing the bureaucratese in this document correctly, the DoD believed the "former" employees who provide documents to Wikileaks are actually current employees whom, one assumes, could be hunted down and squelched. Or that they are agents working for foreign adversaries. Or both.

The DoD also seems to be worried that the information posted on Wikileaks isn't true. To wit:

... the Wikileaks.org Web site could be used to post fabricated information; to post misinformation, disinformation, and propaganda; or to conduct perception management and influence operations designed to convey a negative message to those who view or retrieve information from the Web site....